1. Field of the Invention
The present invention relates in general to well-logging operations, and is particularly useful in anisotropic formations.
2. Description of the Prior Art
In oil and gas exploration, measurement-while-drilling or wireline systems are utilized to develop logs of the earth formations surrounding the borehole. In general, logging packages include: a transmission system that energizes the earth formations with either an electromagnetic field or currents, and at least one receiver system for monitoring the effect that the earth formations (and borehole) have on the field or current.
The electrical properties of the earth formations provide information about the geologic materials that make up the formations, and about their likely oil, gas, and water content. The most useful electrical attribute is resistivity (or conductivity). However, the dielectric property of the earth formations is also of interest.
Some earth formations are isotropic, which means that the earth formation does not preferentially conduct electricity in any particular direction. Other formations are anisotropic, which means that the earth formation may have a greater conductivity in one particular direction than in another particular direction. This is particularly true in many sedimentary geologic formations in which current flows more easily in the direction parallel to the bedding planes than transversely to them due to the fact that a number of mineral crystals are flat or elongated in shape, and they naturally took an orientation parallel to the plane of sedimentation when they were deposited.
In oil and gas exploration, the anisotropy of earth formations can be expressed as a horizontal conductivity, and a vertical conductivity which is transverse. Alternatively, an anisotropy coefficient (xcex) can be utilized to quantify the anisotropy in accordance with Equation No. 1
EQUATION NO. 1:
The determination of horizontal and vertical conductivities is complicated by directional drilling which generates boreholes that are at an angle relative to the earth formations. This angle is called relative dip angle and is often not known precisely.
Others have proposed techniques for determining conductivity in anisotropic formations. In an article by T. Hagiwara entitled xe2x80x9cA New Method to Determine Horizontal-Resistivity in Anisotropic Formations Without Prior Knowledge of Relative Dipxe2x80x9d, presented on Jun. 16-19, 1996, at the SPWLA 37th Annual Logging Symposium, a technique is proposed which allows for the calculation of horizontal conductivity without any prior knowledge of the dip angle. This may reduce the dependence of logging-while-drilling operations on survey instruments.
One problem with the approach suggested by Hagiwara is that influence of the dielectric properties of the formation is largely ignored. In relatively high resistivity geologic formations (like oil-bearing sands) the impact of the dielectric effect increases in influence on the measurements from which conductivity is calculated. Also, the dielectric effect has a greater impact for high frequency interrogation signals.
It is one objective of the present invention to allow for the simultaneous measurement of horizontal and vertical resistivities, horizontal and vertical dielectric permitivity (or xe2x80x9cconstantxe2x80x9d), and the dip angle using multiple frequency resistivity tools and using anisotropic formation models that relate dielectric permitivity to conductivities.
Alternatively, if the dip angle is known, the conductivities and dielectric constants can be measured using multifrequency tools without any anisotropic formation model. The measurements can also be used to verify the validity of the anisotropic formation models which relate horizontal and vertical formation conductivities to the dielectric permitivity. This is not practical with single frequency tools.
Alternatively, if the model which relates horizontal and vertical conductivities to the dielectric effect is known, this invention also allows one to obtain the horizontal resistivity, as well as the horizontal dielectric permitivity, using single frequency tools without knowing the dip angle in high resistivity zones or with high frequency interrogation signals. If the dip angle is known, the vertical resistivity and dielectric permitivity can also be measured.